Study: Tradeoffs needed in battling urban heat island effects

A
team of researchers from Arizona State University have found that
warming resulting from megapolitan expansion is seasonally dependent,
with greatest warming occurring during summer and least during winter.
Among the most practical ways to combat urbanization-induced warming—the
painting of building’s roofs white—was found to disrupt regional
hydroclimate, highlighting the need for evaluation of tradeoffs
associated with combating urban heat islands (UHI).

“We
found that raising the reflectivity of buildings by painting their
roofs white is an effective way of reducing higher average temperatures
caused by urban expansion,” said Matei Georgescu, an assistant professor
in ASU’s School of Geographical Sciences and Urban Planning. “However,
increased reflectivity also modifies hydroclimatic processes and, in the
case of the ‘Sun Corridor,’ can lead to a significant reduction of
rainfall. Our maximum Sun Corridor expansion scenario leads to a 12%
reduction in rainfall, averaged across the entire state. Painting roofs
white leads to an additional 4% reduction in rainfall.”

The
research is presented in the paper, “Seasonal hydroclimatic impacts of
Sun Corridor expansion,” published in the Sept. 7, 2012 issue of
Environmental Research Letters. Georgescu, the lead author of the paper,
is joined by Alex Mahalov, The Wilhoit Foundation Dean’s Distinguished
Professor in the School of Mathematical and Statistical Sciences at ASU,
and Mohamed Moustaoui, an associate professor in ASU’s School of
Mathematical and Statistical Sciences.

The
paper focuses on Arizona’s Sun Corridor, the most rapidly growing
megapolitan area in the United States. Located in a semi-arid
environment, the Sun Corridor is composed of four metropolitan areas:
Phoenix, Tucson, Prescott and Nogales. With a population projection
expected to exceed 9 million people by 2040, the rapidly expanding
megapolitan offers the opportunity to identify tradeoffs focused on
sustainable expansion of the built environment.

The
authors utilized 2050 projections of Sun Corridor growth developed by
the Maricopa Association of Governments (MAG), the regional agency for
metropolitan Phoenix that provides long-range and sustainably oriented
planning. They conducted continuous multi-year, multi-member,
continental scale numerical experiments for several 2050 Sun Corridor
growth and adaptation scenarios and compared results with a modern day
Sun Corridor representation.

“For
a maximum expansion scenario, we find greatest warming to occur during
summer, in excess of 1 C (1.8 F) when averaged over the entire state of
Arizona. Warming remains considerable during both spring and fall
seasons, approaching 0.9 C. For a minimum expansion scenario, the
consistent theme of maximum warming during summer with reduced, although
still significant, warming during spring and fall seasons persists,”
Georgescu added.

Whereas
previous research has documented the contribution of cool roofs as an
effective UHI mitigation approach, this work emphasizes the need to
broadly evaluate impacts by exploring consequences that extend to
hydrology and rainfall.

“Truly
sustainable development will have to consider impacts extending beyond
average temperature,” Georgescu explained. “A crucial step in that
approach is to identify potential adaptation and mitigation strategies
and assess tradeoffs, to ensure that we make smart decisions with
minimum damaging consequences.”

All three co-authors are affiliated with ASU’s College of Liberal Arts and Sciences and the Global Institute of Sustainability.